The present invention relates to injection molding machines for synthetic thermoplastic materials, and more particularly to improvements in apparatus for regulating the speed of, and the magnitude of fluid pressure acting upon, the ram of the injection cylinder in an injection molding machine.
An injection molding machine comprises two basic assemblies, namely a clamping unit which carries the sections of a partible mold and includes means for moving the sections toward or away from each other as well as for maintaining the sections in engagement with each other during injection of plasticized material and subsequent cooling, and an injection unit which causes plasticized synthetic thermoplastic material to flow into the cavity or cavities of the closed mold and applies the pressure which is necessary to insure that each cavity is completely filled with plasticized material as well as that the pressure is maintained during cooling and resulting contraction of shaped article(s). As a rule, the clamping forces and the forces which are needed to move the ram of the injection unit are produced by a highly pressurized hydraulic fluid medium, normally oil. The injection unit is associated with or embodies a plasticizing unit which receives granular plastic material from a feeder and converts it into a flowable mass which is thereupon injected into the mold in response to a forward stroke of the ram in the injection cylinder. The The pressure of fluid which moves the ram in the injection cylinder must be regulated with a high degree of accuracy and reproducibility in order to insure uniform filling of each cavity with plasticized material as well as that the material in each cavity is maintained under requisite pressure during hardening and resulting conversion into a shaped article. The pressure which is to be maintained during setting depends on the characteristics (shrinkage) of thermoplastic material.
The essential parameters in the operation of an injection molding machine, and more particularly in the operation of the injection unit, include the injection pressure, the pressure during setting of injected material, the speed of forward movement of the ram, and the screw back pressure. In presently known apparatus for regulating the admission of pressurized hydraulic fluid into the injection cylinder of an injection molding machine, the fluid flows through a directional control valve having a reciprocable spool. The leakage of fluid in such valves does not allow for accurate and reproducible regulation of fluid pressure in the chamber of the injection cylinder and/or for accurate regulation of the speed of the ram which is used to inject plasticized material into the closed mold. This is due to the fact that the spool does not engage a seat when it is supposed to prevent the flow of fluid, i.e., the land or lands of the spool merely extend into cylindrical portions of the bore in the valve body.